Investigation on electronic structures and nature of charge-transfer transition of ZnO:Co with variation of Co content

A series of Co-doped ZnO are obtained by thermal decomposition of the oxalate precursors. Co ions are in the 2+ formal oxidation states and substituted Zn in ZnO. The band gap energy Eg and the absorption threshold ECT of the charge-transfer transition related to Co2+ decrease with the increasing Co concentration x, but the energy differences between them are almost invariable for all Co-doped ZnO. Eg and ECT reduction with x are mainly due to the valence band raised, and the energy of the acceptor-type ionization levels relative to the conduction band edge is independent of Co content.